This invention relates to an improved sample-and-hold unit whereby the diaphony or crosstalk phenomenon between sequentially memorized signal samples can be reduced.
The sample-and-hold unit according to the invention is particularly adapted to memorize successive electric voltage samples within a very wide dynamic range of variations such as those produced, for example, by a multiplexer heading a seismic data acquisition chain.
For reasons which will be explained below, in the following description, the sample-and-hold units currently in use do not provide for the successive memorization of two samples without occurrence of interaction there between. Accordingly, the memorized value of a sample amplitude is not independent from the memorized value of the amplitude of the preceding sample. The separating power or capability of a sample-and-hold unit is defined as the rate of diaphony between successive samples. This rate never exceeds -80 dB whereas it would be necessary to obtain a rate of -100 dB for example in seismic acquisition chains.
A known method for overcoming the performance limitations of the sample-and-hold units presently in use, consists of omitting them and adapting the following part of the acquisition chain by taking into account the fact that the amplitude of the input signal is always changing. The multiplexer output is connected permanently to the sample amplifier, the amplitude of each sample, varies within the sampling period and, consequently, it is required that the gain program controlling the variation of the amplifier gain, take this variation into account so as to anticipate its probable variation. It thus becomes necessary to measure the signal derivative or the slope of its representing curve and to make use of a gain selection algorithm whereby said gain can be decreased. This algorithm is necessarily more complex than that usable in an acquisition chain comprising a sample memorizing element in which only a uniform gain increase is considered. Since said algorithm is operational in the form of an assembly of logic cabled elements, it results that the sample amplifier provided with such assembly is necessarily bulkier and consumes more power. These disadvantages are particularly troublesome when the data acquisition chain must be housed in a limited space not readily accessible, such as inside a seismic streamer.